1. Technical Field
This application relates to computer storage devices, and more particularly to the field of transferring data between storage devices.
2. Description of Related Art
Host processor systems may store and retrieve data using a storage device containing a plurality of host interface units (host adapters), disk drives, and disk interface units (disk adapters). Such storage devices are provided, for example, by EMC Corporation of Hopkinton, Mass. and disclosed in U.S. Pat. No. 5,206,939 to Yanai et al., U.S. Pat. No. 5,778,394 to Galtzur et al., U.S. Pat. No. 5,845,147 to Vishlitzky et al., and U.S. Pat. No. 5,857,208 to Ofek. The host systems access the storage device through a plurality of channels provided therewith. Host systems provide data and access control information through the channels to the storage device and the storage device provides data to the host systems also through the channels. The host systems do not address the disk drives of the storage device directly, but rather, access what appears to the host systems as a plurality of logical disk units. The logical disk units may or may not correspond to the actual disk drives. Allowing multiple host systems to access the single storage device unit allows the host systems to share data stored therein.
The storage device may include global memory that may be used for data caching. In some cases, the global memory may be distributed among local memory of the plurality of disk adapters, host adapters, and similar devices (generally known as “directors”). However, if a host that is coupled to a particular host adapter accesses cached data in global memory, it is most efficient if the portion of the global memory in which the data is cached corresponds to the local memory of the particular host adapter. However, in most situations, this will not be the case. In some instances, it is more likely that the data will be cached in local memory for a disk adapter coupled to a physical disk containing the data.
Accordingly, it is desirable to provide a system in which data is cached efficiently so that a device, such as a host, that is coupled to a director, accesses cached data from local memory of the director.